The present invention relates to energy storage devices using cell separators, particularly to a separator between stacked energy storage cells using carbon electrodes, and more particularly to an improved multi-cell energy storage device using a cell separator for interconnecting cells containing carbon foam electrodes, whereby the electrodes of adjacent cells are interconnected by a separator composed of a layer of electrically conductive, but ionically isolating material.
Many applications of batteries, fuel cells, and capacitors, such as electric vehicles, medical, etc., require higher voltages than those achieved with a single cell. Using multiple, individually interconnected packaged cells is not desirable because it increases the weight, decreases volumetric packing efficiency, increases costs, and can potentially lead to higher resistances.
Prior efforts have been directed to using multiple cells interconnected and packaged for use in higher voltage energy storage devices, including batteries, electrochemical double-layer capacitors, and fuel cells. For example, double-layer capacitors are well known energy storage devices, see Vols. 1 and 2 of the conference proceedings of "An International Seminar on Double Layer Capacitors and Similar Energy Storage Devices", December 1991 and December 1992. More recently single and multi-cell capacitors have been developed using carbon foam electrodes, as described and claimed in copending U.S. application Ser. No. 07/822,438, filed Jan. 17, 1992, now U.S. Pat. No. 5,260,855, entitled "Supercapacitors Based on Carbon Foams", which allows individual cells to be stacked and interconnected with low electrical resistance and high reliability, while improving packaging efficiency. This is accomplished through the use of cell separators.
Critical requirements for cell separators include long life, low material cost, tolerance to a wide variety of electrolytes, low electrical resistance, and ionic-isolation. The cell separators suitable for use in the multi-cell energy storage devices of above-referenced U.S. application Ser. No. 07/822,438, consisted of metals, conductive rubbers or plastics, non-porous carbon or metal-plastic composites. While these cell separators must be highly electrically conductive and stable chemically with respect to the electrolytes used, while preventing ionic flow but permitting electrical conduction between the cells, it is preferably that the separators be lightweight and have low density to minimize its weight contribution to the energy storage devices.
While the cell separators utilized in the energy storage devices of above-referenced U.S. application Ser. No. 07/822,438, satisfies the above-listed critical requirements, there exists a need to further maximize packaging efficiency and further reduce the costs of multi-cell assembly while meeting these critical requirements. It has been discovered that by interconnecting carbon electrodes of adjacent cells using an appropriate adhesive or paste, a plurality of individual cell units can be connected to form a bipolar-stack energy storage device. Initial testing has established that the above-reference critical requirements have been satisfied while the construction of cell separators for multiple-cell energy storage devices has been greatly simplified. This has been accomplished by a methodology for interconnecting adjacent carbon foam electrodes, which enables stacking of repeating cells to achieve higher voltages for energy storage applications. Thus this invention comprises an improvement over cell separators of the multi-cell energy storage devices of the above-reference patent application.